1. Field of the Invention
This invention relates to an optical transmission system for analog or digital signals, and in particular to a directly modulated solid-state laser. More particularly, the invention relates to the use of chirp modulation using an external modulator to increase the data rate of the optical signal.
2. Description of the Related Art
Directly modulating the analog intensity of a light-emitting diode (LED) or semiconductor laser with an electrical signal is considered among the simplest methods known in the art for transmitting analog signals, such as voice and video signals, on optical fibers. Although such analog transmission techniques have the advantage of substantially smaller bandwidth requirements than digital transmission, such as digital pulse code modulation, or analog or pulse frequency modulation, the use of amplitude modulation may suffer from noise and nonlinearity of the optical source.
For that reason, direct modulation techniques have been used in connection with 1310 nm lasers where the application is to short transmission links that employ fiber optic links with zero dispersion. For applications in metro and long haul fiber transmission links, the low loss of the link requires that externally modulated 1550 nm lasers be used, but such external modulation techniques are complex and expensive.
Direct modulation of lasers at 1550 nm is known for use in digital optical transmission systems such as dense wavelength division multiplexing (DWDM) systems. See, for example, Kartalopoulos, DWDM Networks, Devices, and Technology (IEEE Press, 2002).
One of the issues in designing systems at 1550 nm is that suitable low chirp lasers for use at 1550 nm are not known in the prior art. One type of low chirp laser is the external cavity laser, which is used in digital optical transmission systems, and is a commercially available product. In addition to the low chirp characteristics required for an analog optical transmission system at 1550 nm, the system must be highly linear. Distortion inherent in the operating characteristics of semiconductive lasers prevents a linear electrical modulation signal from being converted linearly to an optical signal, and instead causes the signal to become non-linear or distorted. These effects are particularly detrimental to multi-channel video transmission, which requires excellent linearity to prevent channels from interfering with each other. A highly linearized analog optical system has wide application in commercial analog systems, such as broadcast TV transmission, CATV, interactive TV, and video telephone signal transmission.
The increasing demand for higher data rates and greater throughput in optical fiber networks has created increased attention on a variety of techniques to modulate and encode digital data signals for transmission on optical fiber. One technique called wavelength division multiplexing (WDM) is the use of multiple wavelengths to carry multiple signal channels and thereby greatly increase the capacity of transmission of optical signals over the installed fiber optic networks. See, for example, Kartalopoulos, DWDM Networks, Devices, and Technology (IEEE Press, 2002).
In a WDM optical system, light from several lasers, each having a different central wavelength, is combined into a single beam that is introduced into an optical fiber. Each wavelength is associated with an independent data signal through the optical fiber. At the exit end of the optical fiber, a demultiplexer is used to separate the beam by wavelength into the independent signals. In this way, the data transmission capacity of the optical fiber is increased by a factor equal to the number of single wavelength signals combined into a single fiber.
In the optical transceiver, demultiplexing devices are typically designed to selectively direct several channels from a single multi-channel input beam into separate output channels. Multiplexing devices are typically designed to provide a single multi-channel output beam by combining a plurality of separate input beams of different wavelengths. A multiplexing/demultiplexing device operates in either the multiplexing or demultiplexing mode depending on its orientation in application, i.e., depending on the choice of direction of the light beam paths through the device.
In prior art WDM systems, data carrying capacity may be increased by adding optical channels. Conceptually, each wavelength channel in an optical fiber operates at its own data rate. In fact, optical channels can carry signals at different speeds. In current commercial systems, the use of WDM can push total theoretical capacity per fiber to 160 channels at 25 GHz channel spacing, or 1.6 terabits per second (1.6 tbps). Generally, more space is required between wavelength channels when operating at 10 per second than at 2.5 per second, but the total capacities are nonetheless impressive. For example, in the case of four wavelength channels at a data rate per channel of 2.5 Gigabits per second, a total rate of 10 Gigabits per second is provided. Using eight wavelength channels at a data rate per channel of 2.5 Gigabits per second, a total data rate of 20 Gigabits per second is attained. In fact, other wavelength channels can include, for example, 16, 32, 40 or more wavelength channels operating at 2.5 Gigabits per second or 10 Gigabits per second and allow much higher data throughput possibilities. Furthermore, it is also known in the prior art to use multiple optical fibers in a single cable or conduit can provide even higher transmission rates in a point to point link.
Although high throughput telecommunications networks do not constrain the size of the optical transceiver, optical transceivers for data center applications that use the Ethernet data communications protocol generally conform to IEEE 802.3 standard specifications and MSA form factors. Ethernet (the IEEE 802.3 standard) is the most popular data link network protocol. The Gigabit Ethernet Standard (IEEE 802.3) was released in 1998 and included both optical fiber and twisted pair cable implementations. The 10 Gb/sec Ethernet standard (IEEE 802.3 ae) was released in 2002 with both optical fiber and twisted pair cabling.
The 10 Gigabit Ethernet Standard specifications are set forth in the IEEE 802.3 ae supplement to the IEEE 802.3 Ethernet Standard are currently the highest data rate that has been standardized under the IEEE 802.3 framework. The supplement extends the IEEE 802.3 protocol and MAC specification therein to an operating speed of 10 Gb/s. Several Physical Coding Sublayers known as 100GBASE-X, 10GBASE-R and 10-GBASE-W are specified, as well as a 10 Gigabit Media Independent Interface (XGMII), a 10 Gigabit Attachment Unit Interface (XAUI) and a 10 Gigabit Sixteen-Bit Interface (XSBI) and management
Regardless of whether the system provides for one optical channel, or a WDM system with multiple optical channels, there is interest in increasing the information carrying capacity of an optical channel. Although various techniques such as PAM and QAM are known in the prior art, it has not been known to utilize the jitter or inherent chirp associated with the transmitter to carry an additional information channel or bit.
Prior to the present invention, there has not been an application of an external modulator coupled to the monitor to the output of a directly (current) modulated laser for the purpose of modifying the chirp with an information-containing signal.